The present invention is directed to powder compacts formed from tungsten powder without the aid of organic binders to impart green strength.
Tungsten powders are typically formed into sintered parts by first cold isostaticly forming a compact of the part and then sintering the compact to final density. According to one technique, prior to cold isostatic pressing, the powder may be mixed with an organic binder to impart green strength to the compact so it can be handled before sintering. According to another technique, it is also known that high cold isostatic pressing, on the order of 35,000 lbs/sq. inch can result in a compact having sufficient green strength for subsequent handling. Although lower pressures have been utilized, it is difficult to achieve consistent results at low pressures with certain tungsten powders.
More particularly, commercially available M-55 tungsten powder commercially available from the Chemical and Metallurgical Division, GTE Product Corporation, Hawes Street, Towanda, Pa. 18848, may be compacted at pressures over 35,000 psi. However, it is difficult to compact the powder if the pressure used only reaches 18,000 psi. Since some customers have presses that can produce only 18,000 psi, any improvement which lowers the pressures needed for compaction would be desirable.
It is known to wash tungsten powder in acids such hydrofluoric acid to remove the surface impurity layers from the powder particles to facilitate the process of sintering and formation of stronger bonds between the powder particles. See Institute of Technical Physics, Hungarian Academy of Sciences, Institute of Metallurgy, Academy of Sciences of the USSR, Translated from Poroshkovaya Metallurgiya, No. 6(294), pp. 4-11, Jun. 1987, Original article submitted Mar. 12, 1986. Also, according to U.S. Pat. No. 4,101,309 to Isamu et al, it is known to wash tungsten powders with hydrochloric acid and hydrofluoric acid for obtaining a material, such as a wire, having a low high temperature deformation.
Heretofore, typical prior processes for making substantially pure tungsten powder compacts without the aid of organic binders to impart green strength have utilized relatively high pressures during isostatic pressing of the tungsten powder to achieve consistent results with difficult to compact tungsten powders.